1. Field of the Invention
The present invention relates to a circuit and method for providing a regulated power source to provide power to a DC-DC converter and to charge a Ni-Cad battery.
2. Description of the Prior Art
An AC adapter converts AC line voltage to DC voltage to provide regulated power to various electronic devices, including portable computers. If the device includes a Ni-Cad battery, the AC adapter must provide enough power to operate the device and to charge the Ni-Cad battery. A portable computer uses a DC-DC switching converter to convert the single DC voltage provided by the AC adapter into the multiple DC voltages needed by the electrical circuits within the computer. In this configuration, the Ni-Cad battery and the DC converter are, for the most part, electrically in parallel with each other. The AC adapter provides the raw power to charge the Ni-Cad battery and to operate the DC-DC converter, and the DC-DC converter converts the raw power to the necessary power needed by the portable computer. If the AC adapter is removed, the Ni-Cad battery provides the necessary power to operate the DC-DC converter.
It is desirable, especially in portable computers, to keep the size and weight of the AC adapter as small and light-weight as possible. The physical size, weight and power rating of the AC adapter is proportional to the power that is processed by the AC adapter.
The AC adapter must generate enough power for the DC-DC adapter, taking into account its efficiency, to guarantee that the Ni-Cad battery is charging during its entire charging cycle and to guarantee that the Ni-Cad battery is charged in a desired time. A DC-DC converter is essentially a constant power device, assuring a regulated output voltage and a substantially constant output current. The input voltage of the DC-DC converter can vary, within a specified range, as long as the input current varies inversely so that the resulting input power is sufficient to operate the DC-DC converter. Also, the DC-DC converter has a certain efficiency rating such that the power available to the computer from the DC-DC converter is somewhat less than the power applied at the input of the DC-DC converter.
The Ni-Cad battery requires a certain amount of energy at all times while connected to ensure that it is being charged. This is especially important at the beginning of a charge cycle where the Ni-Cad battery voltage is near the minimum required voltage for the DC-DC converter. If the AC adapter does not supply enough power, the Ni-Cad battery might be temporarily discharged, which could pull the voltage down below the voltage needed by the DC-DC converter. Further, the Ni-Cad batteries must be charged at a sufficient rate so that they are fully charged from a discharged state in a desired amount of time.
The AC adapter must be designed to sustain the proper voltage and current requirements to ensure that the Ni-Cad battery, if connected, will be charging at all times and in sufficient time. The voltage of a discharged Ni-Cad battery is low and rises as it is charged until it reaches its maximum voltage level. The voltage supplied by the AC adapter must always be at least as large as the Ni-Cad battery voltage so that the battery is always being charged. Likewise, the AC adapter must be able to provide more current than the DC-DC converter needs at any given voltage level within the specified voltage range. Once the Ni-Cad battery is fully charged, a trickle current must be maintained through the battery to keep it charged.
The conventional AC adapter was designed to provide output voltage regulation with a maximum current limit. This conventional design required that the regulated output voltage of the AC adapter had to be at least as great as the maximum Ni-Cad battery voltage when fully charged to prevent discharging of the battery. Furthermore, the conventional AC adapter had to be able to provide at least the maximum current drawn by the DC-DC converter when a discharged Ni-Cad battery was inserted into the system, plus enough extra current to guarantee that the battery began charging. The voltage of the Ni-Cad battery controls the input voltage to the DC-DC converter, such that the DC-DC converter draws more current in order to meet its input power requirements when a discharged Ni-Cad battery is installed. Therefore, the lowest value for the maximum current limit was approximately equal to the maximum current required by the DC-DC converter (at the minimum voltage level) plus enough extra current to guarantee some battery charge current. The conventional AC adapter thus had to have a minimum power rating of the regulated output voltage multiplied by the maximum current limit.
Even if the low voltage battery charge current was set at minimal levels, the minimum power rating, as derived from the conventional method of designing the AC adapter, however, is significantly larger than the theoretical power needed by the DC-DC converter and the Ni-Cad battery, both at a worst case instant and cumulatively over the desired charge time period, to meet the continuous charging and charge time period requirements. Consequently, the physical size and weight of the conventional AC adapter was significantly larger than theoretically necessary since its size and weight was proportional to its power rating.
Consider, for example, a DC-DC converter which requires an input power of twenty watts. Also, assume that the Ni-Cad battery needs an average of ten watts of power to fully charge it within the desired amount of time. Theoretically, then, an AC adapter need only provide approximately thirty watts of power to operate the DC-DC converter and to charge the Ni-Cad battery at the minimum desired rate. Assume that the Ni-Cad batteries in the example range from 10 volts when discharged to 18 volts when fully charged. Since a discharged Ni-Cad battery would pull the voltage down to ten volts at the beginning of the charge cycle, the DC-DC converter needs about two amperes of current in order to meet its input power requirement of twenty watts. The conventional AC adapter would provide a small additional amount of current above the two amperes in order to guarantee that the battery starts to charge in order to avoid discharging. Therefore the current limit is set at just over 2 amperes. Eventually, the Ni-Cad battery voltage reaches eighteen volts so that the conventional AC adapter had to regulate its output voltage to at least eighteen volts. The conventional AC adapter, therefore, had to have a power rating of at least eighteen volts times two amperes, which equals thirty six watts. Recall that theoretically, only thirty watts was necessary to achieve this task. The power rating of the conventional AC adapter was, therefore, at least twenty percent greater than necessary, which made it approximately twenty percent larger than necessary, all other things being equal.